As devices using compound semiconductors are known field effect devices and bipolar devices, such as InP-HEMT (High Electron Mobility Transistor), InP-HBT (Heterojunction Bipolar Transistor), etc. Specifically, InP-HEMT is applied to signal processing circuits of the optical communication and high-speed digital circuits of others because of the high-speed characteristic. InP-HEMT is expected to be applied also to amplifiers of the microwave band and the milliwave band because of the low noise characteristic.
The interconnection among such devices using compound semiconductors is generally provided by forming an insulating film over a semi-insulating substrate and forming Au interconnections thereover. In MMIC (Microwave Monolithic Integrated Circuit), etc. requiring operations at higher frequencies, the coplanar interconnection structure is used in more cases so as to match the impedance between interconnections.
In such interconnection, as the operation frequency is higher, the loss due to the interconnection resistance and the transmission loss of the signal due to the dielectric loss are easily generated.
Background arts are disclosed in, e.g., Japanese published unexamined patent application No. 2000-91426, Japanese published unexamined patent application No. 2001-223331, Japanese published unexamined patent application No. Hei 11-145386, Japanese published unexamined patent application No. Hei 9-162285, Japanese published unexamined patent application No. Hei 10-242717, Japanese published unexamined patent application No. Hei 11-017467 and Japanese published unexamined patent application No. 2002-190545.
To generate no dielectric loss, the state that no dielectric is present around the interconnections is ideal. Especially, it will be effective that no semiconductor substrate is present.
In the usual compound semiconductor device, a semi-insulating semiconductor substrate is used as the substrate. However, depending on the use purpose, application, etc. of the device, an n-type conductive semiconductor substrate is often used. In this case, because of the conductivity of the substrate, the signal is attenuated, and the transmission characteristics are undesirably deteriorated.
In Japanese published unexamined patent application No. 2000-91426, etc., cavities are provided on the side of the substrate so as to improve the transmission characteristics, etc. However, the technique disclosed in Japanese published unexamined patent application No. 2000-91426, etc. cannot form cavities which are large enough to decrease the dielectric loss, and if large cavities are formed, there will be a risk that the ceilings of the cavities would collapse.